In the design of shoes, in particular sports shoes, there are a number of contradicting design goals to be realized. On the one hand, a sports shoe should cushion the loads arising on the body and be capable of permanently resisting the arising forces. On the other hand, a sports shoe should be lightweight in order to hinder, as little as possible, the course of movement of the athlete.
Known sports shoes typically use foamed materials in the sole area to meet the above described requirements. For example, foams made out of ethylene vinyl acetate (EVA) have deformation properties that are well suited for cushioning ground reaction forces. Using different densities and modifying other parameters, the dynamic properties of such foams can be varied over wide ranges to take into account the different loads in different types of sports shoes, or in different parts of a single sports shoe, or both.
Shoe soles with foamed elements, however, have a number of disadvantages. For example, the cushioning properties of an EVA foam depend significantly on the surrounding temperature. Further, the lifetime of a foamed cushioning element is limited. Due to the repeated compressions, the cell structure of the foam degrades over time, such that the sole element loses its original dynamic properties. In the case of running shoes, this effect can occur after approximately 250 km. In addition, manufacturing a shoe with foamed sole elements having different densities is so costly that shoes are often produced only with a continuous midsole made from a homogeneous EVA-foam. The comparatively high weight is a further disadvantage, in particular with hard foams having greater densities. Further, sole elements of foamed materials are difficult to adapt to different shoe sizes since larger designs can result in undesired changes of the dynamic properties.
It has, therefore, been tried for many years to replace known foamed materials with other sole constructions that provide similar or better cushioning properties at a lower weight, where the sole constructions are unaffected by temperature, can be cost-efficiently produced, and have a long lifetime. For example, German Patent Application Nos. DE 41 14 551 A1, DE 40 35 416 A1, DE 102 34 913 A1, and DE 38 10 930 A1, German Utility Model No. DE 210 113 U, and European Patent No. EP 0 741 529 B1, the entire disclosures of which are hereby incorporated herein by reference, disclose constructions of this type. The foam-free sole designs of the prior art, however, have until now not gained acceptance. One reason is that the excellent cushioning properties of EVA foams have not been sufficiently achieved in these foam-free designs. This applies in particular for the heel area where the ground reaction forces acting on the sole reach their maximum values, which can exceed several times the weight of an athlete.
It is, therefore, an object of the present invention to provide a shoe sole that can be cost-efficiently manufactured and provide good cushioning properties in a heel area without using foamed materials so that, if desired, the use of a foamed material is no longer necessary.